Mechanical integrity of polysilicon films exposed to hydrofluoric acid solutions

Abstract

This paper presents the results from a comparative study of Young’s modulus, residual stress, and membrane burst pressure of undoped LPCVD polysilicon films exposed to various concentrations of hydrofluoric acid (HF). Load deflection measurements on square membranes of polysilicon with residual tensile stress were used to obtain estimates of Young’s modulus, residual stress and burst pressure. The polysilicon membranes were exposed to four different solutions of the 49% by weight reagent HF including 10:1 DI water and HF, 1:1 DI water the HF, commercial 10:1 buffered oxide etchant, or pure HF (i.e. 49% by weight reagent). Two control groups were studied composed of membranes with no treatment and membranes exposed to DI water. Young’s modulus changed from an average of 190 GPa for the control groups to an average of 240 GPa for films exposed to pure HF. Residual stress values exhibited a less pronounced and opposite change, with an average of 42 MPa for the control groups and an average of 27 MPa for films exposed to pure HF. Similarly, burst pressure was seen to decrease with increasing HF concentration, ranging in value from an average of 96.5 kPa (14 psi) for the control groups to an average of 34.5 kPa (5 psi) for films exposed to pure HF. It was found that the change in the investigated mechanical properties of polysilicon was approximately equal for HF:DI solutions of HF concentration above 10%. Furthermore, for solutions of equal HF concentration, the addition of the buffering agent decreases the effect on membrane burst pressures significantly.